In surgical and field emergency situations it is commonly necessary to introduce an endotracheal tube to a patient to aid or permit respiration. It is necessary to determine that the tube has been properly introduced into the trachea and not the esophagus. In surgical settings, the quantitative measurement of carbon dioxide by capnometers during the respiratory cycle has been used to confirm proper tube placement. P. K. Birmingham et al., "Esophageal Intubation", Anesth. Analy, 1986, 65, 865-91. Capnometers are not considered suitable for emergency field use because they are expensive and not reliable under extreme ambient conditions or when subjected to rough handling.
Colorimetric indicators responsive to CO.sub.2 have been connected to endotracheal tubes to provide a qualitative indication of CO.sub.2 in the tube that indicates the correct location of the tube (see U.S. Pat. Nos. 4,691,701 and 4,728,499). However, these devices are single patient use, subject to the user's color perception and must be directly observed at all times to verify respiration.
Fertig et al., U.S. Pat. No. 4,595,016 discloses an Apnea monitor in which a continuous breath sample is drawn through the cell of an infrared analyzer comprising a capacitor microphone detector responsive to CO.sub.2 in exhaled breath. The capacitor microphone generates a signal each time it detects an exhaled breath. When a timer senses an extended delay between signals, an alarm sounds indicating the presence of an Apnea condition. Capacitor microphone detectors are vibration sensitive and therefore subject to damage by field use handling.